Composite panel having a thermoplastic seam weld

ABSTRACT

The present invention is a method for joining thermoplastic composite sandwich panels with thermoplastic welds (fusion bonds) made without autoclave processing of the joint. The preferred joint is a double interleaf staggered joint with supporting titanium doublers providing a tensile strength of at least 12,000 lb/in. The joint is particularly suited for joining sections of a cryogenic tank for spacecraft.

REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional application based uponU.S. patent application Ser. No. 09/729,794, filed Feb. 23, 2001, whichwas a divisional application based upon U.S. patent application Ser. No.09/120,500, filed Jul. 21, 1998, now U.S. Pat. No. 6,284,089, whichclaimed the benefit of U.S. Provisional Patent Application 60/068,719,filed Dec. 23, 1979.

TECHNICAL FIELD

[0002] The present invention relates to a process for joining honeycombsandwich panels with thermoplastic welds, particularly double interleafstaggered joints connecting the face sheets of the respective panels.

BACKGROUND ART

[0003] The use of composites in primary structure in aerospaceapplications is limited today because of the relatively high cost. Asignificant contribution to the total cost is the assembly cost wherethe precured composite elements are assembled, drilled, and fastened.The necessary design for mechanical fastening complicates the structure,especially in thin sections, because of the need for access to bothsides of the bond line.

[0004] While composites might be adhesively bonded, cocured, or welded,these connecting processes generally produce bonds that rely upon theresin matrix for strength. The bond line lacks any reinforcing materialto help with load transfer. These bonds generally have modest strength,and are susceptible to disbanding with shock impact or other “out ofplane” forces affecting the assembly. Such forces often arise inenvironments prone to vibration.

[0005] 1. Composite Manufacturing

[0006] Fiber-reinforced organic resin matrix composites have a highstrength-to-weight ratio (specific strength) or a highstiffness-to-weight ratio (specific stiffness) and desirable fatiguecharacteristics that make them increasingly popular as a replacement formetal in aerospace applications where weight, strength, or fatigue iscritical. Thermoplastic or thermoset organic resin composites would bemore economical with improved manufacturing processes that reduced touchlabor and forming time.

[0007] Prepregs combine continuous, woven, or chopped reinforcing fiberswith an uncured matrix resin, and usually comprise fiber sheets with athin film of the matrix. Sheets of prepreg generally are placed(laid-up) by hand or with fiber placement machines directly upon a toolor die having a forming surface contoured to the desired shape of thecompleted part or are laid-up in a flat sheet which is then draped andformed over the tool or die to the contour of the tool. Then the resinin the prepreg lay up is consolidated (i.e. pressed to remove any air,gas, or vapor) and cured (i.e., chemically converted to its final formusually through chain-extension or fused into a single piece) in avacuum bag process in an autoclave (i.e., a pressure oven) to completethe part.

[0008] The tools or dies for composite processing typically are formedto close dimensional tolerances. They are massive, must be heated alongwith the workpiece, and must be cooled prior to removing the completedpart. The delay caused to heat and to cool the mass of the tools addssubstantially to the overall time necessary to fabricate each part.These delays are especially significant when the manufacturing run islow rate where the dies need to be changed frequently, often afterproducing only a few parts of each kind. An autoclave has similarlimitations; it is a batch operation.

[0009] In hot press forming, the prepreg is laid-up to create a preform,which is bagged (if necessary), and placed between matched metal toolsthat include forming surfaces to define the internal, external, or bothmold lines of the completed part. The tools and composite preform areplaced within a press and then the tools, press, and preform are heated.

[0010] The tooling in autoclave or hot press fabrication is asignificant heat sink that consumes substantial energy. Furthermore, thetooling takes significant time to heat the composite material to itsconsolidation temperature and, after curing the composite, to cool to atemperature at which it is safe to remove the finished composite part.

[0011] As described in U.S. Pat. No. 4,657,717, a flat composite prepregpanel was sandwiched between two metal sheets made from asuperplastically formable alloy and was formed against a die having asurface precisely contoured to the final shape of the part.

[0012] Attempts have been made to reduce composite fabrication times byactively cooling the tools after forming the composite part. Theseattempts have shortened the time necessary to produce a composite part,but the cycle time for heating and cooling remains long. Designing andmaking tools to permit their active cooling also increases their cost.

[0013] Boeing described a process for organic matrix forming andconsolidation using induction heating in U.S. Pat. No. 5,530,227. There,Boeing laid up prepregs in a flat sheet sandwiched between aluminumsusceptor sheets. The susceptor sheets were susceptible to heating byinduction and formed a retort to enclose the prepreg preform. To ensurean inert atmosphere around the preform during curing and to permitwithdrawing volatiles and outgassing during the consolidation, the facesheets are welded around their periphery. Such welding unduly increasedthe preparation time and the cost for part fabrication. It also ruinedthe susceptor sheets (i.e., prohibited their reuse) which added asignificant cost penalty to each part fabricated with this approach.Boeing described in U.S. Pat. No. 5,599,472 a technique that readily andreliably sealed the susceptor sheets of the retort without the need forwelding and permitted reuse of the susceptor sheets in certaincircumstances. This “bag-and-seal” technique applies to both resincomposite and metal processing.

[0014] 2. Processing in an Induction Press

[0015] The dies or tooling for induction processing in Boeing'sinduction heating workcell are ceramic because a ceramic is notsusceptible to induction heating and, preferably, is a thermal insulator(i.e., a relatively poor conductor of heat). Ceramic tooling usually isstrengthened and reinforced internally with fiberglass rods or otherappropriate reinforcements and externally with metal or other durablestrongbacks to permit it to withstand the temperatures and pressuresnecessary to form, to consolidate, or otherwise to process the compositematerials or metals. Ceramic tools cost less to fabricate than metaltools of comparable size and have less thermal mass than metal tooling,because they are unaffected by the induction field. Because the ceramictooling is not susceptible to induction heating, it is possible to embedinduction heating elements in the ceramic tooling and to heat thecomposite or metal retort without significantly heating the tools. Thus,induction heating can reduce the time required and energy consumed tofabricate a part.

[0016] While graphite or boron fibers can be heated directly byinduction, most organic matrix composites require a susceptor in oradjacent to the composite material preform to achieve the necessaryheating for consolidation or forming. The susceptor is heatedinductively and transfers its heat principally through conduction to thepreform or workpiece that, in Boeing's prior work, is sealed within thesusceptor retort. Enclosed in the metal retort, the workpiece does notexperience the oscillating magnetic field which instead is absorbed inthe retort sheets. Heating is by conduction from the retort to theworkpiece.

[0017] Induction focuses heating on the retort (and workpiece) andeliminates wasteful, inefficient heat sinks. Because the ceramic toolsin the induction heating workcell do not heat to as high a temperatureas the metal tooling of conventional, prior art presses, problems causedby different coefficients of thermal expansion between the tools and theworkpiece are reduced. Furthermore, Boeing's induction heating press isenergy efficient because significantly higher percentages of inputenergy go to heating the workpiece than occurs with conventionalpresses. The reduced thermal mass and ability to focus the heatingenergy permits change of the operating temperature rapidly whichimproves the products produced. Finally, the shop environment is notheated as significantly from the radiation of the large thermal mass ofa conventional press. The shop is a safer and more pleasant environmentfor the press operators.

[0018] In induction heating for consolidating and forming organic matrixcomposite materials, Boeing generally places a thermoplastic organicmatrix composite preform of PEEK or ULTEM, for example, within the metalsusceptor envelope (i.e., retort). These thermoplastics have a lowconcentration of residual volatile solvents and are easy to use. Thesusceptor face sheets of the retort are inductively heated to heat thepreform. Consolidation and forming pressure consolidate and, ifapplicable, form the preform at its curing temperature. The sealedsusceptor sheets form a pressure zone in the retort in a manneranalogous to conventional vacuum bag processes for resin consolidation.The retort is placed in an induction heating press on the formingsurfaces of dies having the desired shape of the molded composite part.After the retort and preform are inductively heated to the desiredelevated temperature, differential pressure (while maintaining thevacuum in the pressure zone around the preform) across the retort whichfunctions as a diaphragm in the press forms the preform against the dieinto the desired shape of the completed composite panel.

[0019] The retort often includes three, stacked susceptor sheets sealedaround their periphery to define two pressure zones. The first pressurezone surrounds the composite panel/preform or metal workpiece and isevacuated and maintained under vacuum. The second pressure zone ispressurized (i.e., flooded with gas) at the appropriate time to helpform the composite panel or workpiece. The shared wall of the threelayer sandwich that defines the two pressure zones acts as thediaphragm.

[0020] Boeing can perform a wide range of manufacturing operations inits induction heating press. These operations have optimum operatingtemperatures ranging from about 350° F. (175° C.) to about 1850° F.(1010° C.) or even above. For each operation, Boeing usually holds thetemperature relatively constant for several minutes to several hours tocomplete the operations. While temperature can be controlled bycontrolling the input power fed to the induction coil, a better andsimpler way capitalizes on the Curie temperature. Judicious selection ofthe metal or alloy in the retort's susceptor face sheets avoidsexcessive heating irrespective of the input power. With improved controland improved temperature uniformity in the workpiece, Boeing producesbetter products. The method capitalizes on the Curie temperaturephenomenon to control the absolute temperature of the workpiece and toobtain substantial thermal uniformity in the workpiece by substantiallymatching the Curie temperature of the susceptor to the desiredtemperature of the induction heating operation being performed. TheCurie temperature is generally slightly above the processingtemperature. With thermoplastic welding, for example, it might be themelt temperature of the matrix resin plus about 1-75° F. (preferably5-25° F.) so that the bond line remains in the processing window withoutexcessive heating. This temperature control method is explained ingreater detail in Boeing's U.S. Pat. Nos. 5,723,849 or 5,645,744.

[0021] U.S. Pat. No. 5,587,098 describes joining large structures in aBoeing induction heating press with localized heating along the bondline. The die is modified to include a “smart” susceptor correspondingto the region of the bond line. The susceptor heats when the embeddedinduction coil is energized. The temperature of the susceptor iscontrolled by its Curie point. The susceptor heats the bond lineselectively, and the surrounding ceramic dies trap the heat. Residualstress at the joint can be relieved by using a susceptor that hassegments of different Curie temperature materials extending in asuccessive pattern outwardly from the bond line to produce a thermalgradient from ambient to the bond line temperature. We prefer two orthree temperature zones with these segments, typically a central weldingor bonding zone with outer zones each 50-100° F. lower in temperaturethan the welding zone.

[0022] 3. Thermoplastic Welding

[0023] Three major joining technologies exist for aerospace compositestructure: mechanical fastening; adhesive bonding; and welding. Bothmechanical fastening and adhesive bonding are costly, time consumingassembly steps that introduce excess cost even if the parts that areassembled are fabricated from components produced by an emerging, costefficient process. Mechanical fastening requires expensive holelocating, drilling, shimming, and fastener installation, while adhesivebonding often requires complicated surface pretreatments.

[0024] Thermoplastic welding eliminates fasteners. It joinsthermoplastic composite components at high speeds with minimum touchlabor and little, if any, pretreatments. A conventional weldinginterlayer tape (compromising the susceptor and surroundingthermoplastic resin either coating the susceptor or sandwiching it) alsocan simultaneously take the place of shims required in mechanicalfastening. As such, composite welding promises to be an affordablejoining process. For “welding” a combination of thermoplastic andthermoset composite parts together, the resin that the susceptor meltsfunctions as a hot melt adhesive. If fully realized,thermoplastic-thermoset bonding in addition to true thermoplasticwelding will further reduce the cost of composite assembly.

[0025] There is a significant stake in developing a successful inductionthermoplastic welding process. Its advantages versus traditionalcomposite joining methods are:

[0026] reduced parts count versus fasteners

[0027] minimal surface preparation, in most cases a simple solvent wipeto remove surface contaminants

[0028] indefinite shelf life at room temperature

[0029] short process cycle time, typically measured in minutes

[0030] enhanced joint performance, especially hot/wet and fatigue

[0031] the possibility of rapid field repair of composites or otherstructures.

[0032] There is little or no loss of bond strength after prolongedexposure to environmental influences.

[0033] U.S. Pat. No. 4,673,450 describes a method to spot weld graphitefiber reinforced PEEK composites using a pair of electrodes. Afterroughening the surfaces of the prefabricated PEEK composites in theregion of the bond, Burke placed a PEEK adhesive ply along the bondline, applied a pressure of about 50-100 psi through the electrodes, andheated the embedded graphite fibers by applying a voltage in the rangeof 20-40 volts at 30-40 amps for approximately 5-10 seconds with theelectrodes. Access to both sides of the assembly was required in thisprocess which limited its application.

[0034] U.S. Pat. Nos. 3,966,402 and 4,120,712 describe thermoplasticwelding with induction heating. In these patents, conventional metallicsusceptors are used and have a regular pattern of openings oftraditional manufacture. Achieving a uniform, controllable temperaturein the bond line, which is crucial to preparing a thermoplastic weld ofadequate integrity to permit use of welding in aerospace primarystructure, is difficult with those conventional susceptors.

[0035] Thermoplastic welding is a process for forming a fusion bondbetween two faying thermoplastic faces of two parts. A fusion bond iscreated when the thermoplastic on the surface of the two thermoplasticcomposite parts is heated to the melting or softening point and the twosurfaces are brought into contact, so that the molten thermoplasticmixes. The surfaces are held in contact while the thermoplastic coolsbelow the softening temperature.

[0036] The same process parameters apply essentially to hot meltthermoplastic adhesive bonds between prefabricated thermoset composites.

[0037] Simple as the thermoplastic welding process sounds, it isdifficult to perform reliably and repeatably in a real factory onfull-scale parts to build a large structure such as an airplane wingbox. One difficulty is heating the bond line properly withoutoverheating the entire structure. Another difficulty is achievingintimate contact of the faying surfaces of the two parts at the bondline during heating and cooling because of (1) the normal imperfectionsin the flatness of composite parts, (2) thermal expansion of thethermoplastic during heating to the softening or melting temperature,(3) flow of the thermoplastic out of the bond line under pressure (i.e.,squeeze out), and (4) contraction of the thermoplastic in the bond lineduring cooling.

[0038] The exponential decay of the strength of magnetic fields withdistance from their source dictates that, in induction weldingprocesses, the susceptible structure closest to the induction coil willbe the hottest, since it experiences the strongest field. Therefore, itis difficult to obtain adequate heating at the bond line between twographite or carbon fiber reinforced resin matrix composites relying onthe susceptibility of the fibers alone as the source of heating in theassembly. For the inner plies to be hot enough to melt the resin, theouter plies closer to the induction coil and in the stronger magneticfield are too hot. The matrix resin in the entire piece of compositemelts. The overheating results in porosity in the product, delamination,and, in some cases, destruction or denaturing of the resin. To avoidoverheating of the outer plies and to insure adequate heating of theinner plies, we use a susceptor of significantly higher conductivitythan the fibers to peak the heating selectively at the bond line insteadof in the composites themselves. To create a weld, an electromagneticinduction coil heats a susceptor to melt and cure a thermoplastic resin(also sometimes referred to as an adhesive) to bond the elements of theassembly together.

[0039] The current density in the susceptor may be higher at the edgesof the susceptor than in the center because of the nonlinearity of thecoil, such as occurs when using a cup core induction coil like thatdescribed in U.S. Pat. No. 5,313,037. Overheating the edges of theassembly can result in underheating the center, either condition leadingto inferior welds because of non-uniform curing. An open or mesh patternin the susceptor embedded at the bond line allows the resin to createthe fusion bond between the composite elements of the assembly when theresin heats and melts.

[0040] a. Moving coil welding processes

[0041] In U.S. Pat. No. 5,500,511, Boeing described a tailored susceptorfor approaching the desired temperature uniformity. Designed for usewith the cup coil of U.S. Pat. No. 5,313,037, this susceptor relied uponcarefully controlling the geometry of openings in the susceptor (boththeir orientation and their spacing) to distribute the heat evenly. Thesusceptor had a regular array of anisotropic, diamond shaped openingswith a ratio of the length (L) to the width (W) greater than 1. Thissusceptor produced a superior weld by producing a more uniformtemperature than obtainable using a susceptor having a similar array,but one where the L/W ratio was one. Changing the length to width ratio(the aspect ratio) of the diamond-shaped openings in the susceptorproduced a large difference in the longitudinal and transverseconductivity in the susceptor, and, thereby, tailored the currentdensity within the susceptor. A tailored susceptor having openings witha length (L) to width (W) ratio of 2:1 has a longitudinal conductivityabout four times the transverse conductivity. In addition to tailoringthe shape of the openings to tailor the susceptor, Boeing altered thecurrent density in regions near the edges by increasing the foil density(i.e., the absolute amount of metal). Increasing the foil density alongthe edge of the susceptor increased the conductivity along the edge andreduced the current density and the edge heating. Boeing increased foildensity by folding the susceptor to form edge strips of double thicknessor by compressing openings near the edge of an otherwise uniformsusceptor. These susceptors were difficult to reproduce reliably. Also,they required careful placement and alignment to achieve the desiredeffect.

[0042] The tailored susceptor was designed to use with the cup coil ofU.S. Pat. No. 5,313,037, where the magnetic field is strongest near theedges because the central pole creates a null at the center. Therefore,the tailored susceptor was designed to counter the higher field at theedges by accommodating the induced current near the edges. The highlongitudinal conductivity encouraged induced currents to flowlongitudinally.

[0043] The selvaged susceptor for thermoplastic welding which isdescribed in U.S. Pat. No. 5,508,496 controls the current densitypattern during eddy current heating by an induction coil to providesubstantially uniform heating to a composite assembly and to insure thestrength and integrity of the weld in the completed part. This susceptoris particularly desirable for welding ribs between prior welded sparsusing an asymmetric induction coil (described in U.S. Pat. No.5,444,220), because it provides (1) a controllable area of intense,uniform heating under the poles of the coil; (2) a trailing region withessentially no heating; and (3) a leading region with minor preheating.

[0044] Boeing achieved better performance (i.e., more uniform heating)in rib welding by using a selvaged susceptor having edge strips withoutopenings. The resulting susceptor, then, has a center portion with aregular pattern of openings and solid foil edges, referred to as selvageedge strips. The susceptor is embedded in a thermoplastic resin to makea susceptor/resin tape that is easy to handle and to use in preformingthe composite pieces prior to welding. Also, with a selvaged susceptor,the impedance of the central portion should be anisotropic with a lowertransverse impedance than the longitudinal impedance. Here, the L/Wratio of diamond shaped openings should be less than or equal to one.With this selvaged susceptor in the region immediately under theasymmetric induction work coil, current flows across the susceptor tothe edges where the current density is lowest and the conductivity,highest.

[0045] Generally, the selvaged susceptor is somewhat wider than normalso that the selvage edge strips are not in the bond line. Boeingsometimes removes the selvage edge strips after forming the weld,leaving only a perforated susceptor foil in the weld. This foil has arelatively high open area fraction.

[0046] Another difficulty remaining in perfecting the thermoplasticwelding process for producing large scale aerospace structures in aproduction environment involved control of the surface contact of thefaying surfaces of the two parts to be welded together. The timing,intensity, and schedule of heat application must be controlled so thematerial at the faying surfaces are brought to and maintained within theproper temperature range for the requisite amount of time for anadequate bond to form. Intimate contact is maintained while the meltedor softened material hardens in its bonded condition.

[0047] Large scale parts, such as wing spars and ribs, and the wingskins that are bonded to the spars and ribs, are typically on the orderof 20-30 feet long at present, and potentially as much as 100 feet inlength when the process is perfected for commercial transport aircraft.Parts of this magnitude are difficult to produce with perfect flatness.Instead, the typical part will have various combinations of surfacedeviations from perfect flatness, including large scale waviness in thedirection of the major length dimension, twist about the longitudinalaxis, dishing or sagging of “I” beam flanges, and small scale surfacedefects such as asperities and depressions. These irregularitiesinterfere with full surface area contact between the faying surfaces ofthe two parts and actually result in surface contact only at a few “highpoints” across the intended bond line. Applying pressure to the parts toforce the faying surfaces into contact achieves additional surfacecontact, but full intimate contact is difficult or impossible to achievein this way. Applying heat to the interface by electrically heating thesusceptor in connection with pressure on the parts tends to flatten theirregularities further, but the time needed to achieve full intimatecontact with the use of heat and pressure is excessive, can result indeformation of the top part, and tends to raise the overall temperatureof the “I” beam flanges to the softening point, so they begin to yieldor sag under the application of the pressure needed to achieve a goodbond.

[0048] Boeing's multipass thermoplastic welding process described inU.S. Pat. No. 5,486,684 (which we incorporate by reference) enables amoving coil welding process to produce continuous or nearly continuousfusion bonds over the full area of the bond line. The result is highstrength welds produced reliably, repeatably, and with consistentquality. This process produces improved low cost, high strengthcomposite assemblies of large scale parts fusion bonded together withconsistent quality. It uses a schedule of heat application thatmaintains the overall temperature of the structure within the limit inwhich it retains its high strength. Therefore, it does not requireinternal tooling to support the structure against sagging whichotherwise could occur when the bond line is heated above the highstrength temperature limit. The process also produces nearly completebond line area fusion on standard production composite parts having theusual surface imperfections and deviations from perfect flatness. Themultipass welding process (1) eliminates fasteners and the expense ofdrilling holes, inspecting the holes and the fasteners, inspecting thefasteners after installation, sealing between the parts and around thefastener and the holes; (2) reduces mismatch of materials; and (3)eliminates arcing from the fasteners.

[0049] In the multipass process, an induction heating work coil ispassed multiple times over a bond line while applying pressure in theregion of the coil to the components to be welded, and maintaining thepressure until the resin hardens. The resin at the bond line is heatedto the softening or melting temperature with each pass of the inductionwork coil and pressure is exerted to flow the softened/melted resin inthe bond line and to reduce the thickness of the bond line. The pressureimproves the intimacy of the faying surface contact with each pass toimprove continuity of the bond. The total time at the softened or meltedcondition of the thermoplastic in the faying surfaces is sufficient toattain deep interdiffusion of the polymer chains in the materials of thetwo faying surfaces throughout the entire length and area of the bondline. The process produces a bond line of improved strength andintegrity in the completed part. Dividing the time that the fayingsurfaces are at the melting temperature allows time for the heat in theinterface to dissipate without raising the temperature of the entirestructure to the degree at which it loses its strength and begins tosag. The desired shape and size of the final assembly is maintained.

[0050] A structural susceptor includes fiber reinforcement within theweld resin to alleviate residual tensile strain otherwise present in anunreinforced weld. This susceptor includes alternating layers of thinfilm thermoplastic resin sheets and fiber reinforcement (usually wovenfiberglass fiber) sandwiching the conventional metal susceptor that isembedded in the resin. While the number of total plies in thisstructural susceptor is usually not critical, Boeing prefers to use atleast two plies of fiber reinforcement on each side of the susceptor.This structural susceptor is described in greater detail in U.S. Pat.No. 5,717,191.

[0051] The structural susceptor permits gap filling between the weldedcomposite laminates which tailors the thickness (number of plies) in thestructural susceptor to fill the gaps, thereby eliminating costlyprofilometry of the faying surfaces and the inherent associated problemof resin depletion at the faying surfaces caused by machining thesurfaces to have complementary contours. Standard manufacturingtolerances produce gaps as large as 0.120 inch, which are too wide tocreate a quality weld using the conventional susceptors.

[0052] It is easy to tailor the thickness of the structural susceptor tomatch the measured gap by scoring through the appropriate number ofplies of resin and fiber reinforcement and peeling them off. In doingso, a resin rich layer will be on both faying surfaces and this layershould insure better performance from the weld.

[0053] b. Fixed coil induction welding

[0054] Thermoplastic welding using Boeing's induction heating workcelldiffers from the moving coil processes because of the coil design andresulting magnetic field. The fixed coil workcell presents promise forwelding at faster cycle times than the moving coil processes because itcan heat multiple susceptors simultaneously. The fixed coil can reduceoperations to minutes where the moving coil takes hours. The keys to theprocess, however, are achieving controllable temperatures at the bondline in a reliable and reproducible process that assures quality weldsof high bond strength. The fixed coil induces currents to flow in thesusceptor differently from the moving coils and covers a larger area.Nevertheless, proper processing parameters permit welding with theinduction heating workcell using a susceptor at the bond line, asdescribed in U.S. Pat. No. 5,641,422.

[0055] Another advantage with the fixed coil process is that welding canoccur using the same tooling and processing equipment used toconsolidate the skin, thereby greatly reducing tooling costs. Finally,the fixed coil heats the entire bond line at one time to eliminate theneed for shims that are currently used with the moving coil. To controlthe temperature and to protect against overheating, “smart” susceptorsare used as a retort, as the bond line susceptor material, or both.

[0056] The need for a susceptor in the bond line poses many obstacles tothe preparation of quality parts. The metal which is used because of itshigh susceptibility differs markedly in physical properties from theresin or fiber reinforcement so dealing with it becomes a significantissue. A reinforced susceptor overcomes problems with conventionalsusceptors by including the delicate metal foils (0.10-0.20 inchwide×0.005-0.010 inch thick; preferably 0.10×0.007 inch) in tandem withthe warp fibers of the woven reinforcement fabric. The weave fibers holdthe foils in place longitudinally in the fabric in electrical isolationfrom each other yet substantially covering the entire width of the weldsurface. This arrangement still has adequate space between the foils forthe flow and fusion of the thermoplastic resin. Furthermore, in the bondline, the resin can contact, wet, and bond with the reinforcing fiberrather than confronting the resinphilic metal of the conventionalsystems. There will be a resin-fiber interface with only short runs of aresin-metal interface. The short runs are the length of the diameter oftwo weave fibers plus the spatial gap between the weave fibers, which isquite small. Thus, the metal is shielded within the fabric and a betterbond results. In this woven arrangement to foil can assume readily thecontour of the reinforcement. Finally, the arrangement permits efficientheat transfer from the foil to the resin in the spatial region where thebond will form.

[0057] The reinforced susceptor might be an analog of the structural,selvaged, or tailored susceptors of Boeing's other applications (i.e. atape encased in resin and placed along the bond line) or may befabricated as part of the facing plies of the prefabricated compositesthat abut along the bond line.

[0058] The susceptor may be a multistrip susceptor having two or moreparallel foil strips that extend the full length of the strip. The foilis usually about 0.007 inch thick and each strip is about 0.10-0.20 inchwide. The strips are separated by gaps of comparable width or slightlywider dimension which we etch or ablate from a solid foil. Along thelength of the susceptor, periodically, transverse spacer strips span thegap and keep the carrier strips apart. The foil can be virtually anywidth. It can be about two-four inches wide to match the spar cap widthor might even be the full width of sheets of the composite prepreg usedto form the skins. Dimensions given are typical and could be varied.

[0059] The strength and durability of adhesive bonds or thermoplasticwelds connecting composite structure is improved by adding Z-pinmechanical reinforcement to the bond line. Weld strength can also beimproved with a post-weld anneal to control cooling of the bond line.

[0060] 4. Joining Honeycomb Sandwich Panels

[0061] Thermoplastic honeycomb sandwich panels have strength-to-weightratios and use temperature capabilities unmatched by metals or thermosetcomposites. These properties make the panels ideally suited for highperformance aircraft and spacecraft, especially for cryogenic tanks.But, in making large structures, often two or more subparts must bejoined. The joint usually is designed to withstand twice the load of thebulk material, and is impenetrable to liquid and vapor. Traditionaljoints used a butt or single lap splice combined with composite doublersfastened or adhered on each side of the face sheets. The followingproblems characteristically plagued these joints:

[0062] 1. Drilling fastener holes in the graphite reinforcedthermoplastic composite was costly and greatly compromised its loadcarrying capabilities.

[0063] 2. Fastener holes required extensive sealing to ensure liquid andvapor integrity.

[0064] 3. Fasteners added significant weight.

[0065] 4. Film adhesives are restricted to a limited range oftemperatures.

[0066] 5. In highly loaded applications, film adhesives usually age,peel, and crack.

[0067] 6. Costly modifications to core ends (ramping up or down) or theaddition of a backup structure, or both, were often required to supportfastener installation.

[0068] 7. Butt and single lap joints are inherently weak because they donot share fibers between face sheet elements. This discontinuity createsa stress concentration area with limited load carrying capabilities.

[0069] Developing a structural joint that is easily and reliable formedwithout autoclave processing would greatly enhance the application ofthermoplastic composite sandwich panels to large structures. Such jointsneed to be able to withstand tensile stresses in the range of 12,000lb/inch.

SUMMARY OF THE INVENTION

[0070] The present invention is a thermoplastic composite sandwich panelhaving a thermoplastic weld on at least one face sheet made withoutautoclave processing in a double interleaf stagger joint or equivalentconfiguration. The invention eliminates fasteners and produces a fluidimpervious, sealed joint. The panels are particularly suited for use ascryogenic tanks in spacecraft or launch vehicles.

[0071] A double interleaf staggered joint for joining thermoplasticfiber-reinforced composites with a thermoplastic seam weld, has at leasttwo fully compacted, laminated face sheets substantially free ofvolatiles having a plurality of plies of a thermoplastic resinreinforced with fibers. Each face sheet has an edge with a plurality offingers adapted for interleaving into the joint. The face sheets alsohave a right hand and a left hand configuration suitable for forming thejoint. Resin film between fingers on each joint mating interface insuresthat fusion of the fingers produces a resin-to-resin bond. Optionally, ametal foil on one surface of the face sheets in the region of the jointreinforces the joint. Each finger is a plurality of plies offiber-reinforced resin composite with the plies staggered to reduceareas of stress concentration in the joint.

[0072] Our preferred process for making the joint includes the steps of:

[0073] (a) laying up upper and lower, left and right laminated facesheets having a plurality of plies of fiber-reinforced thermoplasticmatrix resin;

[0074] (b) positioning separator plies in the edges of the face sheetsthat will form the joint to create an interleaf split offiber-reinforced laminated composite fingers;

[0075] (c) consolidating the face sheets to produce compositessubstantially free of volatiles or porosity;

[0076] (d) bonding the upper and lower face sheets, respectively, to ahoneycomb core with a resin rich layer on the surface of the face sheetthat contacts the honeycomb core to form left and right honeycomb coresandwich panels;

[0077] (e) removing the separator plies;

[0078] (f) interleaving fingers of the left and right face sheets of theleft and right panels above and below the core to define doublestaggered interleaf joints;

[0079] (g) optionally, positioning a metal foil between each face sheetand the core in the region of the joint to reinforce the joint; and

[0080] (h) melting the resin in the joints to form thermoplastic fusionwelds.

[0081] Our preferred apparatus for forming a fusion weld in a jointbetween interleaved fingers of at least two, preconsolidated,fiber-reinforced thermoplastic resin matrix composite sandwich panels,includes clamping elements adapted for applying a pressure to the joint,and insulation associated with edges of the platen to contain heat atthe joint. The elements including heated, conformal platens matching theconfiguration of the panels.

BRIEF DESCRIPTION OF THE DRAWINGS

[0082]FIG. 1 is a perspective view of a moving coil thermoplasticwelding apparatus.

[0083]FIG. 2 is an exploded view of a typical joint of the presentinvention with a foaming adhesive bond connecting honeycomb core anddouble interleaf staggered joints in the face sheets.

[0084]FIG. 3 is a graph showing the relative weight per unit area forthree alternative aerospace structures: (1) conventional aluminum skinand stringers; (2) graphite/epoxy skin and stringers; and (3) thesandwich panel configuration of the present invention.

[0085]FIG. 4 is a cross-sectional elevation of a cryogenic tank having athermoplastic seam weld of the present invention.

[0086]FIG. 5 is an elevation of a typical double interleaf staggeredjoint in a composite sandwich panel.

[0087]FIG. 6 illustrates a single butt joint with doublers for a facesheet in composite honeycomb sandwich panels.

[0088]FIG. 7 illustrates a staggered butt joint for a face sheet incomposite honeycomb sandwich panels.

[0089]FIG. 8 illustrates a double interleaf joint for a face sheet incomposite honeycomb sandwich panels.

[0090]FIG. 9 is a detail illustrating the features of a preferred doubleinterleaf staggered joint of the present invention.

[0091]FIG. 10 illustrates the process for interleaving left and righthand panels to assemble a joint of the present invention.

[0092]FIG. 11 is a view, similar to FIG. 8, showing the completed doubleinterleaf staggered joint in a composite honeycomb sandwich panel.

[0093]FIG. 12 is a typical processing cycle used to form thethermoplastic seam weld.

[0094]FIG. 13 is an elevation of our preferred joint forming apparatus.

[0095]FIG. 14 illustrates the temperature profile achievable with thejoint forming apparatus of FIG. 13.

[0096]FIG. 15 is a plan view of a typical susceptor for thermoplasticwelding.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0097] First, this section will provide some general discussion aboutthermoplastic welding before turning toward its application for joiningthermoplastic honeycomb sandwich structure.

[0098] 1. Thermoplastic Welding with a Moving Coil

[0099] As shown in FIG. 1, a thermoplastic welding head 10 includesleading and trailing pneumatic pressure pads and a primary inductioncoil between the pads that is supported on tooling headers 12 over thethermoplastic composite parts or details to be fusion bonded (i.e.,welded) together. In the example shown in FIG. 1, the parts include athermoplastic spar 14 and a thermoplastic wing skin 16, only a smallsection of which is shown in FIG. 1. The spar 14 is in the form of an“I” beam having a top cap 18, a bottom cap 20, and a connecting web 22.The wing skin is bonded over the full length and surface area of thespar cap 18 with sufficient strength to resist the tensile and peelingforces the wing will experience in flight. The apparatus shown is morefully described in U.S. Pat. No. 5,556,565. The beams might be allcomposite construction or a hybrid metal webbed composite capped beam asdescribed in U.S. Pat. No. 5,556,565. We could also join thermoset skinsand spars with a hot melt thermoplastic adhesive.

[0100] A copper mesh susceptor 32 (i.e., a metal foil 702 susceptible toinduction heating encapsulated in a thermoplastic resin 704, FIG. 15) isinserted between the spar cap 18 and the wing skin 16. Typically theencapsulating resin is the same or a slightly lower melting temperatureformulation of the same thermoplastic resin of the spar cap 18 and thelower faying surface of the wing skin 16.

[0101] The welding head 10 can be any moving coil apparatus that iscapable of applying pressure during induction heating of the bond lineto promote fusion and after heating for a period sufficient for theresin to cool and harden in its bonded condition. Suitable welding headsare disclosed in U.S. Pat. Nos. 5,635,094; 5,444,220; and 5,313,037. Apreferred welding apparatus uses the induction coil for inducing eddycurrents in the susceptor 32. The eddy currents heat the susceptor byelectrical resistance heating and soften or melt the thermoplastic resinin the faying surfaces of the parts so it flows, interdiffuses, andfuses together with softened resin of the wing skin and spar cap uponcooling.

[0102] The coil shown in U.S. Pat. No. 5,313,037 provides zero eddycurrent at the center with the current density increasing toward theedges. Use of a tailored susceptor is desirable to counterbalance thenonuniform eddy current density that the coil produces from centerlineto edge to achieve uniform heating, and such a susceptor is disclosed inU.S. Pat. No. 5,500,511. A selvaged susceptor designed especially foruse with the asymmetric induction coil of U.S. Pat. No. 5,444,220 isdescribed in U.S. Pat. No. 5,508,496.

[0103] The primary induction coil is mounted in the welding head in thecenter of a lower frame which is pinned to a link connecting the lowerframe to an upper frame. The upper frame is pulled by a motive apparatusincluding a stepper motor driving a drive sprocket and a chain loopthrough a reduction gear unit. A pair of camroll bearings projects fromboth sides of the lower frame into cam grooves milled into the insidesurfaces of the headers to guide and support the lower frame. A similarset of camroll bearings projects outward from the upper frame into astraight cam groove to guide the upper frame as it is pulled by thechain loop from one end of the wing skin to the other.

[0104] The process of welding the wing skin to the spar cap begins withassembling the parts together with the susceptor 32 interposed betweenthe faying surfaces of the parts. In the case of a wing box, we attachthe susceptor 32 to the outer surfaces of the spar caps 18 and 20 andthen sandwich the spars between the upper and lower wing skins 16. Theparts are held in position and squeezed together by a force exerted by apair of air bearing pads. Pressurized air is delivered with air linesand is distributed to the air bearing pressure pads by separate airlines. The air to the pads reduces the frictional drag on the pressurepads on the top surface of the wing skin and helps to cool the partsafter the coil has passed. The induction coil moves along the intendedbond line over the outer surface of the wing skin in general alignment(±0.125 in) with the susceptors. The moving coil produces an alternatingmagnetic field which projects through the wing skins and around thesusceptor, generating eddy currents in the susceptor. The eddy currentsare of sufficient amperage to heat the susceptor, raising thetemperature of the thermoplastic material in the faying surfaces to itssoftening or melting temperature. After the first pass of the weldinghead over each bond line to seal the box, the process is repeated threeor more times, usually increasing the power to the coil after the secondpass and, if desired, increasing the pressure exerted by air cylinderson the pressure pads.

[0105] The bond strength improves with multiple passes of the weldinghead over the same bond line. Multiple passes of the induction coilserves to create the optimal conditions for achieving a fusion bond withthe desired characteristics of continuity over the entire bond line, andsubstantial molecular interdiffusion of the materials in the fayingsurfaces to produce a bond line of high pulloff strength with thecomplete or nearly complete absence of voids, as discussed in U.S. Pat.No. 5,486,684. Welds having higher pulloff strengths use a barbedsusceptor on the bond line.

[0106] The mechanisms for achieving a fusion bond include intimatecontact and “healing.” Intimate contact of the two faying surfaces is afunction of force exerted on the parts to squeeze them together, andtemperature-dependent viscosity. The force exerted on the parts isdistributed over a certain surface area as interfacial pressure tendingto bring the faying surfaces together. The viscosity of the surfacematerial is manifested by the tendency of high spots in the surface toyield of flow so that low spots in the two surfaces can come together.“Healing” is partly a process in which molten or softened materials flowtogether and blend where they come into contact, and partly a process ofmolecular penetration of the polymer chains in the material of onesurface into the molecular matrix of the material in the other fayingsurface. The average penetration distance of the polymer chains, withoutthe beneficial mixing effect achieved by flowing the materials in thefaying surfaces, increases as a quarter power (the fourth root) of time(i.e., t^(0.25)).

[0107] Objective and easily made observations of a bond line that areindicative of “healing” of the quality of the bond are reduction in bondline thickness, improved ratio of bonded to unbonded surface area in thebond line (or expressed conversely, a reduction of the amount ofunbonded surface area in the bond line), and improved pass-through of abonding resin through openings in the susceptor.

[0108] Irregularities, such as hollows, depressions, and asperities(i.e., peaks) in the faying surfaces of the parts, and other deviationsfrom perfect flatness can interfere with and prevent continuous intimatecontact along the full surfaces of the parts where bonding is intended.Other deviations from perfect flatness, include scratches and bumps andlarge scale features such as waviness in the direction of the majorlength dimension, twist about the longitudinal axis, dishing or saggingof “I” beam flanges, and warping such as humping or bowing in thelongitudinal direction. The structural susceptor is particularly suitedfor dealing with these problems.

[0109] Boeing's goal is to produce aircraft structure that eliminatesfasteners. Welded structure will be far less expensive because weldingeliminates the labor to drill holes accurately and to inspect thefasteners after installation. We also will avoid other problems thatfasteners introduce, such as sealing around the fastener and the holes,mismatch of materials, and arcing from the fasteners. To replace thefasteners, however, requires confidence that the welds are uniform andconsistent. A failure at any weak point in the weld could lead tocatastrophic unzipping of the entire welded structure. An importantproblem with quality welding is temperature uniformity along the bondline to achieve uniform and complete melt and cure of the resin. Being a“smart” susceptor, the barbed susceptor has a Curie temperature slightlyhigher than the welding temperature (i.e., about 700° F.) so thepossibility of disastrous overheating is reduced.

[0110] Boeing embeds the foil in the resin to simplify the weldingprocess. Making a foil/resin tape eliminates the steps of applyingseparate layers of resin between the respective elements in acomposite-susceptor-composite assembly. It also ensures that there willalways be adequate resin proximate the susceptor and essentially uniformresin thickness across the welding bond line. The typical tape is about2-4 inches wide with K3A Avimid resin (an aromatic polyimide), althoughthe resin can be PEEK, PEKK, PES, PEK, ULTEM, or any otherthermoplastic. The resin must be compatible with the matrix resin in thecomposite and generally is the same resin as the matrix resin whenwelding thermoplastic composites. For the “welding” analog for thermosetcomposites, the resin will likely be a comparable thermoplasticformulation of the matrix resin in the composites or a compatible resin.

[0111] The welding process might cause the assembly to sag when the bondline reaches the melt temperature where the resin flow needed forforming the fusion bond occurs. Supporting the assembly from the insidewill prevent sagging. If support tooling is used, the part design mustallow removal of the support tooling after the welds are formed. Forexample, the assembly cannot have completely closed cavities. The needto remove the support tooling can severely impact the parts that we canfabricate.

[0112] The integrity of the weld is critical to the performance of thecompleted, welded structure. The quality of the weld is related to thetemperature along the bond line and good welds require control of thetemperature within a relatively narrow range during the welding. Weparticularly want to avoid overheating, so a “smart” susceptor made froma Co, Ni, or Fe alloy with a Curie temperature slightly above themelting temperature of the resin will help ensure that we producequality welds. By “slightly above” we mean within about a processingwindow for the resin where a weld will form but the resin will notdenature or pyrolyze and the composite will not delaminate. This windowextends from the melt temperature (T_(g)) to about +75° F. above themelt temperature. Furthermore, an alloy like INVAR42 (42% Ni-58% Fe) hasa coefficient of thermal expansion (CTE) comparable to the resincomposite so that embedding the susceptor into the completed part willnot have as dramatic an impact if the susceptor is such an alloy ratherthan copper or another metal where the CTE mismatch between the resinand susceptor is larger.

[0113] Suitable thermoplastic resins include polyimides, PEEK, PEK,PEKK, PES, PPS, TORLON (i.e. PEI), or the like. It is especially suited,however, for consolidation or forming of resins that have low volatilescontent and that are nonreactive (i.e., the true thermoplastics likePEEK or ULTEM).

[0114] In conventional thermoplastic welding, the susceptor is aseparate element and may be in sheet, mesh, expanded, milled, selvagedor other suitable form. The susceptor should be structured for theoptimum conductivity longitudinally and transversely needed to obtaincontrolled, reliable, and reproducible heating. Geometry and structureare closely related to the type of induction head used, as those ofordinary skill will understand. In U.S. Pat. No. 5,916,460, Woolley andScoles integrated the susceptor into the detail part along the bondline. Integrated, the susceptor still needs to have all the favorableproperties of the conventional, separate susceptor.

[0115] Throughout this discussion, we use “composite” to mean a fiberreinforced organic resin matrix material. The fibers should be ofsuitable strength to make aerospace structural parts, such as graphite,fiberglass, or carbon fibers. The organic resin can be a thermosettingresin, such as epoxy or bismaleimide, or a thermoplastic, such as ULTEMor K3B polyimide, as we previously described.

[0116] A spar detail part might include a stubble surface of Z-pins sothat a padup strip includes pins extending upwardly from the panel aswell as downwardly from the spar flange. The pins might be carried in apadup strip as Pannell suggested with stubble on both faces with longer,integral pins if the detail parts are thermoplastic rather thaninserting the pins into the spar and panel prior to their curing.

[0117] If welding and Z-pinning, Boeing prefers to use pins in thedetail parts that penetrate further into the parts than the region whichsoftens during the formation of a fusion bond between the details. Inthis way, the pins stay firmly anchored in their desired orientation. Wecan heat the bond line through the susceptor with induction orresistance heating.

[0118] As previously mentioned, the susceptor typically comprises ametal mesh encased in a resin. The susceptor often includes selvage edgestrips. The mesh includes a repeating pattern of diamond shaped openingsof length (L) and width (W) separated by fine-line elements. It isencased in resin to provide better heat transfer, to provide adequateresin for the weld, and to reduce the likelihood of introducing porosityin the weld by filling openings in the susceptor.

[0119] For combining the integrated susceptor and Z-pins, we mightsimply apply a metal foil which the pins pierce on insertion to yield apatterned susceptor. We suspect, however, that it would be difficult toobtain controlled heating of this pierced susceptor reliably from partto part and configuration to configuration. Therefore, we prefer toposition the pins in the pre-existing openings of the expanded foil. Thesusceptor might be fashioned as Kirkwood et al. suggested in U.S. Pat.No. 5,756,973 as a “barbed wire” analog. Alternatively, the susceptormight be of the reinforced design woven and aligned with the associatedfiber in the reinforcing fabric. Then, the fibers will protect thedelicate foil when the pins are inserted.

[0120] Post-weld anneal and its controlled cooldown offer a significantstrength improvement and a higher quality weld. Parts made with ananneal also show lower process variance. The anneal is done about 125°F. below the resin's melt temperature. The strength enhancement achievedwas considerable in the range of about 15-25%. The optimal annealingconditions for all the resin systems or welded assembly configurationsstill need to be determined. The anneal probably achieves some mobilityin the resin or relieves tensile strain otherwise occurring at the bondline. Boeing also has not fully explored the optimal duration of theanneal. Twenty minutes produced the significant strength increases forScoles. Longer or short cycles or even multiple cycles might prove to bemore effective. With combined welding and annealing processes, the bondline cools slowly with a plateau in its cooling at 500° F. Annealing'sbenefits, however, may be achievable with other cooling cycles thatresult in the bond line temperature exceeding 500° F. for the 20 minutesor so of the anneal. That is, the anneal might actually be a slowcooldown over the 20 minute anneal cycle at about 5-6° F./minute fromthe welding temperature of about 620° F. to 500° F.

[0121] 2. The Type of Joint

[0122] Now, turning to the process of the present invention for joininga (with thermoplastic welds) thermoplastic composite sandwich panel, weexplored several joint designs in autoclave tests. The autoclaved jointswere easier to prepare to screen the designs for the highest strengthjoint configuration. We sought an autoclaved joint having a strength of16,000 lb/in, believing that nonautoclave processing would reduce thejoint strength by about 20%. We investigated single butt (FIG. 6),staggered butt (FIG. 7), various interleaf joints, joints reinforcedwith doublers, and joints reinforced with titanium foil. From thesetests we determined that a double interleaf joint (FIG. 8 or 11) was thebest candidate for nonautoclave processing trials. Those subsequenttrials led us to add a resin encapsulated titanium foil (902, FIG. 9) onthe underside of the joint to support the laminate plies 73 duringremelt in the thermoplastic welding (fusion) process. Our preferredjoint is shown in FIG. 5. It includes upper and lower face sheets 502and 504 sandwiching a honeycomb core 506. The face sheets each include adouble interleaf staggered butt joint (FIG. 8) thermoplastic weld. Theface sheets are adhered to the honeycomb core. The core has a foamingadhesive butt joint 508 along the thermoplastic weld bond line of theface sheets.

[0123] The double interleaf joint (FIG. 8) was slightly more difficultto make than a staggered butt joint (FIG. 7), but it was inherentlystronger. The double interleaf autoclaved joint had a strength in excessof 16,000 lb/in for a twelve ply base laminate (K3B/thermoplasticpolyimide resin reinforced with graphite fibers). FIG. 10 shows ourpreferred embodiment for such a double interleaf nonautoclave joint. Theleft layup 150 and right hand layup 200 consist of twelve plythermoplastic composite tape or fabric ply stacks. These stacks aresymmetric about a centerline with each half layup consisting of sixplies. Each ply is staggered typically 0.1 inches away from the previousply. The first six plies of the right hand layup are located below thefirst six plies of the left hand layup with the end ply extending thetotal joint length (2.7 inches in the preferred configuration) away fromthe first ply of the right hand layup. Next, the second six plies of theright hand layup lie on top of the first six plies of the left handlayup with the end ply located 0.1 inches away from the last ply of thefirst six ply right hand layup. Finally, the second six plies of theleft hand layup are located on top of the second six plies of the righthand layup with the end ply residing 0.1 inches away from the last plyof the first six ply left hand layup (FIG. 9).

[0124] The typical ply table for the groups of plies is: Ply Layer Ply(11) & (14) 0   45 60 90 −60 0 Group  (6) & (7) 0 −60 90 60   45 0

[0125] This joint is a gradually ramped double interleaf joint 512 (FIG.11) with a center joint overlap region of about 0.5 inch length. Tosupport the plies of the joint over a honeycomb core, we include a 0.005inch thick chromic acid anodized, titanium foil 902 (FIG. 10) primedwith a K3B thermoplastic polyimide resin slurry and cocured with one plyof 0° K3B composite tape on each side of the foil. Between each leg ofthe interleaf, a layer of 0.005 inch thick thermoplastic resin (K3A orULTEM) to facilitate nonautoclave joining. A 0.010 inch thick optionallayer of thermoplastic resin (K3A or ULTEM) facilitates bonding of thejoint to the honeycomb substructure.

[0126] The benefits of the double interleaf staggered joint are:

[0127] 1. The interleaf creates common fibers that aid in load carryingbetween left and right hand laminate members.

[0128] 2. The staggers within the joint eliminate stress concentrationregions.

[0129] 3. The geometry of the joint lends itself to nonautoclaveprocessing.

[0130] 4. The titanium foil in the joint adds to its strength and shiftsthe joint failure mechanism from the ramp-up area to the foil edge.

[0131] 5. The titanium supports the laminate during nonautoclave fusionoperations.

[0132] 6. Mechanical fasteners and associated holes are not required tocreate the joint.

[0133] 7. The joint is inherently stronger than other similar weightjoint configurations.

[0134] 8. The completed joint is impenetrable to liquid and vapor.

[0135] 9. The joint is only slightly harder to produce than easy buttjoints with doublers, but its strength is dramatically higher.

[0136] 3. Nonautoclave Processing to Form the Joint

[0137] The first step in making joined honeycomb panels involves layingup an upper left, upper right, and lower left, and lower right facesheet 73 using removable separator plies an 75 half way through thestacks to create an interleaf split (see FIG. 10) of laminatedfiber-reinforced fingers. Preconsolidation of the face sheets isrequired prior to honeycomb core assembly, since the temperatures (665°F. for K3B) and pressures (185 psi for K3B) required to fullyconsolidate the thermoplastic are incompatible (will dimple and cookiecut face sheets or crush the core with low density cores (such as ⅜″cell, 3 lb/ft³ density titanium core). Therefore, the face sheet layupsare consolidated in an autoclave using conventional bagging andconsolidation cycles to ensure that the face sheets are fully compactedand are substantially free of volatiles (e.g., prepreg lacquers orsolvents).

[0138] The separator plies (75) should be a thick nonporous release film(such as Armalon) that will not contaminate the joint. Initial attemptsused steel shims, treated with Frekote (a release agent), but the shimsleft a residual film after consolidation. Surface abrading was requiredto insure removal of the residual film.

[0139] The staggered plies should be laid up with a typical dimension ofat least 0.1″/stagger to insure that there are no abrupt drop-offs(hence areas of stress concentration) within the joint. Initially, welaid up face sheets plies without staggers and ground back the plies tocreate a ramp after consolidation. Without the staggers however, sincethe bonding resin was not adhered to the laminate in a high-temperaturehigh-pressure autoclave cycle, the final bond quality suffered.

[0140] A resin film (such as K3A or ULTEM) should be laid up at alljoint mating interfaces to insure that the bonding resin is fused to thebase laminate in the preconsolidation cycle. Thereby, during subsequentnonautoclave fusion steps a resin-to-resin bond is made rather than aresin-to-base laminate bond.

[0141] A titanium foil and bonding resin film are placed under facesheets to aid in subsequent joining of left and right hand coreassemblies.

[0142] The third step in making a thermoplastic/honeycomb joint is todetool the preconsolidated face sheets. The separator plies 75 stay withthe face sheets through this operation. Minor deflashing and abradingmay be required to insure proper mating of the left and right hand facesheets, but no cleanout of the split interior or addition of bondingresin is required prior to joint assembly.

[0143] Then, the preconsolidated face sheets are bonded to the honeycombcore. The face sheets are made with a resin rich layer on the side thatmates to the core to facilitate fusion to a resin infused core. A spacerblock (typically INVAR 42) is used at the bonded core ends to supportthe staggered face sheets and the separator film is left in place fromthe previous step. The face sheet to core bonding autoclave cycle is alower temperature (525° F. for K3B laminates with K3A resin film) andpressure (35 psi for K3B/K3A) cycle than that used to preconsolidate theface sheets to avoid damaging the core or face sheets. This cycle can beat a lower temperature and pressure since a resin (K3A) to resin (K3A)fusion is being made.

[0144] The fifth step in making a thermoplastic/honeycomb joint involvesremoving the spacer blocks and assembling the left and right hand facesheets into the interleaf stagger configuration. Thereafter, without anautoclave, we fuse (i.e., create a thermoplastic weld between) theinterleaved fingers together to form the Double Interleaf joint and buttthe honeycomb together. In most cases a conventional foaming adhesivebetween the mated cores creates a structural joint. This foamingadhesive is usually a thermoset or thermoplastic resin that foams and/orcures as the face sheets are being melted together.

[0145] A description of the apparatus we have developed to makenonautoclave joints is detailed in the following section.

[0146] 4. Nonautoclave Joining Apparatus

[0147] Through a series of process optimization trials, we created ahigh quality thermoplastic/honeycomb joint (for example, K3A as thebonding resin, K3B as the resin in the face sheets and a titanium,honeycomb core by subjecting the double interleaf joint to a 50 psi, 30minute, 650° F. process cycle. This cycle (FIG. 12) was hot enough toremelt the thermoplastic face sheet to honeycomb bond created in aprevious autoclave operation. Therefore, a layer of titanium foil wasplaced under the joint heatup zone to support the face sheets during thefusion cycle. During this nonautoclave joining cycle, the foamingadhesive that creates the core joint will also expand and cure.

[0148] Our preferred joint forming tool is shown in FIG. 13. It clampsand holds the assembled double interleaf composite honeycomb panelstogether while creating a bond line pressure of about 50 psi throughoutthe fusion cycle. This pressure aids the formation of an interdiffusedfusion bond in the resin without excessive squeeze out of resin from thepreconsolidated laminates. The tool uses heated platens 602 to heat (orcool) the joint. The center of the clamping apparatus holds a machinedINVAR or steel plate 606 that is thermally conductive and configured tomatch the surface configuration of the completed joint. Outside edges608 of the platens 606 are insulated with mica board or the like tocontain heat at the joint and to create a proper cooling profile for thejoint. The insulation also ensures that the face sheets 502 and 504 donot remelt outside the fusion zone. Pneumatic or hydraulic cylinders 610connect with the platens 602 through suitable backup structure forimparting the desired pressure on the bond line during the fusion cycle.The tool has matching inner and outer platens to sandwich the bond lineduring the fusion cycle. Heating might be accomplished by fabricatingthe plate from a Curie alloy susceptible to induction heating, asdescribed in Boeing's patents, particularly U.S. Pat. No. 5,587,098,which we incorporate by reference. The typical temperature profile weachieve in the fusion zone is shown in FIG. 14.

[0149] Benefits of the nonautoclave joining apparatus are:

[0150] 1. The device can precisely control temperature and pressurethroughout the fusion cycle.

[0151] 2. A variable temperature zone is established that is sufficientto melt and fuse the double interleaf joint, but does not damage thelaminate-to-face sheet bond.

[0152] 3. The platens are shaped so that the joint has the desired,final configuration.

[0153] 4. The apparatus can be constructed to fabricate joints ofpractically any desired contour.

[0154] 5. The apparatus can be constructed using electric or oil heatingand pneumatic or hydraulic pressure application.

[0155] By selecting the proper joint configuration and by optimizing thenonautoclave process, high quality joints that consistently meet thegoal of a 12,000 lb/inch tensile capability are obtainable. The design,process cycles, and joining apparatus support a nonautoclave fusing(welding) method that can be used to join large thermoplastic/honeycombpanels. The resulting joints have strength-to-weight characteristicsunmatched by fastened, bonded, or fastened and bonded systems. Inaddition, the double interleaf joint has the inherent advantages that itis impenetrable to liquid and vapor and can be inspectednondestructively by conventional techniques.

[0156] The joint forming apparatus can clamp the entire seam and formthe weld in a single operation. The apparatus might also clamp only asegment of the weld in which case the weld is made in successivesections. Finally, the clamping apparatus might move along the seamcontinuously or in steps guided by appropriate jigs and tools to guidethe clamping apparatus when creating the weld. Susceptors might beburied in the joint along the interfaces between the fingers or withinthe fingers themselves to aid in heating the joints to the weldingtemperature using induction heating, resistance heating, or acombination of these heating methods.

[0157] The double interleaf staggered joint can be reinforced with Z-pinreinforcement added while forming the weld or inserted after formation.The resin films, for example, along the interfaces of the fingers mightinclude Z-pins in a fashion analogous to the technique described in U.S.Pat. No. 5,876,540.

[0158] While we have described preferred embodiments, those skilled inthe art will readily recognize alternatives, variations, andmodifications which might be made without departing from the inventiveconcept. Therefore, interpret the claims liberally with the support ofthe full range of equivalents known to those of ordinary skill basedupon this description. The examples illustrate the invention and are notintended to limit it. Accordingly, define the invention with the claimsand limit the claims only as necessary in view of the pertinent priorart.

We claim:
 1. A composite honeycomb sandwich panel, comprising: (a) ahoneycomb core having a foaming adhesive butt joint defining a bondline; and (b) upper and lower face sheets adhered to the honeycomb core,each face sheet including a thermoplastic weld substantially along thebond line.
 2. The panel of claim 1 further comprising a resin-encasedmetal foil doubler between the face sheet and the honeycomb core tosupport the face sheet during welding of the weld.
 3. The panel of claim2 wherein the metal foil is titanium.
 4. The panel of claim 1 whereinthe weld is a double interleafed staggered joint to reduce areas ofstress concentration in the joint.